Recorders of parameters such as temperature or humidity or both are well known. Typically, a recorder is provided with one or more pens that are directly coupled by suitable links to sensors. A replaceable planar circular chart is used which is mounted for rotation about an axis on a surface of the recorder under one or several pens. The recorder can be entirely mechanically driven, such as with a spring-wound motor to rotate the chart and with the stylus or pen mounted on, for example, a coiled, bimetallic temperature sensor that causes the pen to move along part of a circular path to record a temperature trace on the chart.
Because the pen motion is along a curved path, the chart bears time lines that are correspondingly curved. In this manner, parameter values can be read from a recorded trace, though with some level of difficulty due to the non-linear appearance of the reference time lines. When several different parameters are to be recorded on the same chart, the different traces, though usually of different colors to enhance their visual separation, are off-set in time with respect to each other. This is due to the different pivot arm lengths associated with the pens. As a result, the combination of curved time lines and time off-set between different traces makes these multiple trace charts more cumbersome to analyze.
In practical applications, it is important that recorders can operate reliably in the absence of electrical power. It is important, for example, for insurance purposes to know what the temperature and humidity conditions are in a computer room after a power failure has occurred. Also, when food is transported in a frozen state, it is desirable to have a record of temperature conditions within the transport.
Portability of a flat chart recorder, therefore, is an important characteristic. Fully mechanical recorders are portable but normally do not have sufficiently long operating cycles for many applications. Battery operated flat chart recorders often have practical time limits that are below desirable operating periods. Mechanically moved pens also tend to be inaccurate as well as difficult to provide compensation for.